The present invention relates to a machine fuel delivery system and method, typically, although not exclusively, for aircraft engines.
Vapour trails are artificial clouds that are visible trails of condensed water vapour exhausted by vehicles' engines. They may be formed as warm, moist exhaust gas mixes with ambient air, and arise from the precipitation of microscopic water droplets or, if the air is cold enough, tiny ice crystals. The term “vapour trails” is intended to refer both to condensation trails (that is to say “contrails”) from aircraft and to water and/or ice precipitation in or attributable to the exhaust plumes from engines of other machines and vehicles, such as ships.
It may be undesirable for some ships to produce vapour trails in certain situations. For example, a military ship producing a vapour trail from its exhaust funnels is highly visible from the air and hence much easier to target.
It is understood that, depending on the timescale considered, the climate-warming impact of aircraft exhaust vapour trails and resulting vapour trail-cirrus may be of a magnitude similar to, or perhaps even greater than, that of the CO2 emitted by aircraft, and therefore may represent a significant element of aviation's total climate impact. It is also understood that an aircraft vapour trail, once formed, will persist in ambient air which is supersaturated with respect to ice, leading to greater climate-warming impact as a result of the increase in longevity of the vapour trail.
US2010/0122519 describes the use of ultra-low sulphur aviation fuel as an alternative to conventional fuel to reduce sulphur by-product generation and hence reduce contrail formation. This document emphasises the need to retain the purity of the ultra-low sulphur aviation fuel, and hence the requirement to manage the supply chain which delivers the fuel, and to avoid mixing with other fuels.
The attempted suppression of vapour trail formation through the reduction of exhaust water vapour content through use of a heat exchanger and condenser arrangement (US2008072577A) potentially introduces significant weight into the engine. Furthermore, the weight penalty is incurred throughout the full duration of a flight, even though vapour trail suppression may only be required for a small percentage of the flight time.
Attempted suppression of vapour trail formation through the use of directed electromagnetic energy (US2010132330A) into the engine exhaust plume could incur a weight penalty. Furthermore, the energy required to operate the system could represent a significant portion of the engine power and thus incur a fuel-burn penalty. Further, in military applications, the emission of powerful electromagnetic radiation has the undesirable effect of increasing the aircraft's detectability.
Attempted suppression of vapour trail formation through the use of ultrasound directed into the engine exhaust plume (US2010043443A) may also incur a material weight penalty associated with equipment for generating the required sound levels.
The attempted modification or suppression of vapour trails through the use of chemicals (U.S. Pat. No. 5,005,355A, U.S. Pat. No. 4,766,725A, U.S. Pat. No. 3,517,505A, U.S. Pat. No. 3,517,512A, US2009013591A) injected either into the engine (whether with the fuel or separately from the fuel) or into the exhaust plume presents the prospect of additional pollution, incurs a weight penalty through the need to carry fuel additives with potentially little or no calorific value of their own (in comparison with conventional aviation fuel when burned within the engine), and may present challenges to engine reliability and/or component life.
The attempted hiding of vapour trails through introducing black carbon into the aircraft engine effluent (U.S. Pat. No. 3,289,409A) results in additional emissions of a species (black carbon) which is known to have an environmental warming impact.
The strategy of avoiding regions prone to vapour trail formation and/or persistence through the routing of aircraft around, above and/or below such regions has the disadvantage that it increases workload for air traffic control and/or pilots, reduces airspace capacity and, in the case of routing around regions prone to vapour trail formation or persistence (which can be tens or hundreds of kilometers in horizontal extent), the length of the route followed by the aircraft is increased, resulting in a fuel-burn penalty. Additionally in the case of climbing so as to fly above regions prone to vapour trail formation or persistence, additional fuel is burned to provide the increased thrust necessary to perform the climb. If aircraft are scheduled to fly below regions prone to vapour trail formation or persistence, additional fuel may be burned subsequently if the aircraft is to return to its optimal cruising altitude once the aircraft has passed the avoided region.
In the case either of climbing so as to fly above or of descending so as to fly below regions of air susceptible to vapour trail formation and/or persistence, the aircraft will be required to fly at an altitude that may differ from the optimal cruise altitude given the aircraft's current weight. In other words, the ability of the aircraft to follow an optimal cruise-climb trajectory is hindered by the requirement to change altitude so as to avoid the region of air susceptible to vapour trail formation and/or persistence.
It is understood that the climate warming impact of a vapour trail of a given horizontal extent is determined, at least in part, by its optical depth. Reductions in the number of soot particles emitted per unit mass of fuel burned by an aircraft's engine could reduce the initial optical depth of exhaust vapour trails. Hereafter in this application the number of soot particles emitted per unit mass of fuel burned is termed the “soot emission index”.
Further, it is understood that the production of soot from engine exhausts can be reduced through the reduction or elimination of aromatic and/or other non-paraffinic content in the fuel used. Biofuels are typically low in aromatics and/or other non-paraffinics. However, biofuels are typically much more expensive than conventional fuels and are in extremely short supply. Hence it is undesirable to fuel a vehicle with biofuel throughout its period of operation, especially as the vehicle it powers may operate for much of its time in conditions where vapour trails will not form and/or persist regardless of the fuel used.
Hence a system which reduces the optical depth of young vapour trails, therefore potentially reducing their climate warming impact, whilst optimising the use of more expensive biofuels, is highly desirable.